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How to use forEach(), Array.map() and Array.filter()? (code example)
- 青灯夜游forward
- 2019-11-28 17:01:482056browse
This article will introduce to you the usage of forEach(), Array.map() and Array.filter(). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.
Array.forEach()
The forEach() method calls the function once for each array element (Callback). [Related course recommendations: JavaScript video tutorial]
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>JavaScript Array.forEach()</title>
<body>
<h2>JavaScript Array.forEach()</h2>
<p>为每个数组元素调用一次函数。</p>
<p id="demo"></p>
<script>
var txt = "";
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
document.getElementById("demo").innerHTML = txt;
function myFunction(value, index, array) {
txt = txt + value + "<br>";
}
</script>
</body>
</html>Please note that this function has 3 parameters:
● Element value
● Element index
●The array itself
The above example only uses the value parameter. The example can be rewritten as:
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>JavaScript Array.forEach()</title>
<body>
<h2>JavaScript Array.forEach()</h2>
<p>为每个数组元素调用一次函数。</p>
<p id="demo"></p>
<script>
var txt = "";
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
document.getElementById("demo").innerHTML = txt;
function myFunction(value) {
txt = txt + value + "<br>";
}
</script>
</body>
</html>Array.map()
The map() method works by executing a function on each array element. Create new array. The map() method is a function that does not execute array elements without values. The map() method does not change the original array.
This example multiplies each array value by 2:
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>JavaScript Array.map()</title>
<body>
<h2>JavaScript Array.map()</h2>
<p>通过对每个数组元素执行函数来创建新数组。</p>
<p id="demo"></p>
<script>
var numbers1 = [45, 4, 9, 16, 25];
var numbers2 = numbers1.map(myFunction);
document.getElementById("demo").innerHTML = numbers2;
function myFunction(value, index, array) {
return value * 2;
}
</script>
</body>
</html>Please note that this function has 3 parameters:
● Element value
● Element index
●The array itself
When the callback function only uses the value parameter, the index and array parameters can be omitted:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Array.map()</h2>
<p>通过对每个数组元素执行函数来创建新数组。</p>
<p id="demo"></p>
<script>
var numbers1 = [45, 4, 9, 16, 25];
var numbers2 = numbers1.map(myFunction);
document.getElementById("demo").innerHTML = numbers2;
function myFunction(value) {
return value * 2;
}
</script>
</body>
</html>Array.filter( )
#filter() method creates a new array containing the array elements of the passed test. This example creates a new array from elements with values greater than 18:
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>JavaScript Array.filter()</title>
<body>
<h2>JavaScript Array.filter()</h2>
<p>使用通过测试的所有数组元素创建一个新数组。</p>
<p id="demo"></p>
<script>
var numbers = [45, 4, 9, 16, 25];
var over18 = numbers.filter(myFunction);
document.getElementById("demo").innerHTML = over18;
function myFunction(value, index, array) {
return value > 18;
}
</script>
</body>
</html>Note that this function has 3 parameters:
● Element value
● Element index
●The array itself
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>JavaScript Array.filter()</title>
<body>
<h2>JavaScript Array.filter()</h2>
<p>使用通过测试的所有数组元素创建一个新数组。</p>
<p id="demo"></p>
<script>
var numbers = [45, 4, 9, 16, 25];
var over18 = numbers.filter(myFunction);
document.getElementById("demo").innerHTML = over18;
function myFunction(value) {
return value > 18;
}
</script>
</body>
</html>This article comes from the js tutorial column, welcome to learn!
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